I found Battle for Wesnoth to be entertaining as a game. It's open source, but I don't know about code quality. If you are willing to read code from when dinosaurs roamed the land and sea, try Nethack.

If you only care about reading code, open source or not, I think the first Deus Ex's in game scripting code was visible in the game's editor. (And the rest was the first Unreal engine, whose might also be available for reading nowadays. Or you just read the source of the various Quakes.)